This invention relates to an improved electrophotographic photoconductor.
Recently, organic photoconductive materials, which are relatively inexpensive and cause no enviromental pollution, have been widely used as a photoconductor of electrophotographic copying machines.
As an organic electrophotographic photoconductor, there have been known photoconductive resin type photoconductors, typically a photoconductor comprising polyvinyl carbazole (PVK); charge-transport complex type photoconductors, typically a photoconductor comprising polyvinyl carbazole-2,4,7-trinitro-fluorenone (PVK-TNF); pigment-dispersed type photoconductors, typically a photoconductor in which phthalocyanine dispersed in a binder resin; and function-separated type photoconductors such as a photoconductor consisting essentially of a charge-generating material and a charge-transport material. Of these, the last one is currently attracting high attention.
When a high-photosensitive photoconductor of the function-separate type is applied to the Carlson process, it exhibits a low chargeability and a weak electric charge retention (large dark decay). Further, the chargeability and electric charge retention properties are drastically deteriorated in the course of repeated and continuous use, causing uneven image density and fogging. In addition, toner particles are unfavorably deposited on the background when reverse development is performed.
In general, the chargeability of the high-photosensitive photoconductor is reduced due to the preexposure fatigue. Such fatigue is chiefly caused by the light absorbed by the charge-generating material contained in the photoconductor. It is therefore considered that the longer the period in which the electric charges generated by the light absorption remain in a migrational state in the photoconductor and the greater the number of the generated electric charges, the greater the reduction in the chargeability of the photoconductor. Even if the photoconductor is electrically charged while the electric charges generated by light-absorption are in a residual state, the surface potential is not elevated until the residual electric charges are dissipated. This is because the electric charges at the surface of the photoconductor are neutralized with the residual carriers which migrate in the photoconductor. The rise of surface potential is thus delayed corresponding to the pre-exposure fatigue, so that the apparent surface potential is lowered.
In an attempt to overcome the above shortcomings in the prior art, the following intermediate layers have been proposed: intermediate layers of a cellulose nitrate resin type as disclosed in Japanese Laid-open Patent Application No. 47-6341, 48-3544 and 48-12034; intermediate layers of a nylon resin type as disclosed in Japanese Laid-open Patent Application Nos. 48-47344, 52-25638, 58-30757, 58-63945, 58-95351, 58-98739 and 60-66258; intermediate layers of a maleic acid resin type as disclosed in Japanese Laid-open Patent Application Nos. 49-69332 and 52-10138; and an intermediate layer of a polyvinyl alcohol resin as disclosed in Japanese Laid-open Patent Application No. 58-105155.
In addition, for the purpose of controlling the electric resistance of conventional intermediate layers, intermediate layers incorporated with various electroconductive additives have been proposed. Examples of such intermediate layers include an intermediate layer made of setting-resins in which carbon or a chalcogen material is dispersed as disclosed in Japanese Laid-open Patent Application No. 51-65942; an intermediate layer made of a compound which is thermally polymerized by using an isocyanate hardening agent together with a quaternary ammonium salt as disclosed in Japanese Laid-open Patent Application No. 52-82238; an intermediate layer containing a resistance-controlling agent as disclosed in Japanese Laid-open Patent Application No. 55-1180451; an intermediate layer made of resin in which aluminum oxide or tin oxide is dispersed as disclosed in Japanese Laid-open Patent Application No. 58-58556; an intermediate layer made of resin which contains an organic metal compound as disclosed in Japanese Laid-open Patent Application No. 58-93062; intermediate layers made of resins in which electroconductive particles are dispersed as disclosed in Japanese Laid-open Patent Application No. Nos. 58-93063, 60-97363 and 60-111255; an intermediate layer made of resin in which magnetite is dispersed as disclosed in Japanese Laid-open Patent Application No. 59-17557; intermediate layers made of resins in which powders of both TiO.sub.2 and SnO.sub.2 are dispersed as disclosed in Japanese Laid-open Patent Application Nos. 59-84257, 59-93453 and 60-32054; and an intermediate layer in which indium oxide is dispersed as disclosed in Japanese Laid-open Patent Application No. 57-81269.
The above-mentioned intermediate layers, however, cannot provide sufficient effect on the prevention of the reduction in the chargeability caused by the repeated and continuous use of the photoconductors, the delay in the rise of the surface potential thereof, or the considerable change in the residual electric potential.